1. Field of the Invention
The invention relates to a projection objective for microlithography for imaging a pattern arranged in an object plane of the projection objective into an image plane of the projection objective.
2. Description of the Related Art
Projection objectives of this type are used in projection exposure systems for the production of semiconductor components and other finely structured components, in particular in wafer scanners and wafer steppers. They are used to project patterns of photo masks or graduated plates, which will also be designated masks or reticles in the following text, onto an object coated with a light-sensitive layer at the maximum resolution and on a reducing scale.
In this case, in order to produce finer and finer structures, it is necessary firstly to increase the image-side numerical aperture (NA) of the projection objective and secondly to use shorter and shorter wavelengths, preferably ultraviolet light with wavelengths of less than about 260 nm.
In this wavelength range, only a few adequately transparent materials are available for the production of optical components, in particular synthetic quartz glass and fluoride crystals such as calcium fluoride. By means of intrinsic and/or voltage-induced birefringence, when light passes through these materials, a difference in the path between light having a first component of the electric field strength vector and light having a second component of the electric field strength vector perpendicular to this first component can occur.
Since the Abbé constants of the materials available for this wavelength range lie relatively close to one another, it is difficult to provide purely refractive systems with adequate correction of color errors. Therefore, for high-resolution projection objectives use is predominantly made of catadioptric systems in which, apart from refractive elements, reflective elements, for example concave mirrors and deflection mirrors, are also used. During reflection at such an element, the light is generally influenced as a function of polarization, specifically in such a way that a difference in response can occur between light having a first component of the electrical field strength vector, which oscillates perpendicular to the plane of incidence (s-polarized light) and light having a second component, which oscillates parallel to the plane of incidence (p-polarized light).
The two facts outlined above can contribute to the optical path length traced in the projection objective by the light having a first component of the electrical field strength vector differing from that traced by the light having a second component of the electrical field strength factor perpendicular to the first component. In the image plane, the result is the production of two mutually offset partial images (double images), which results in a worsening of the image contrast.
In catadioptric systems, it has additionally been observed that, under certain imaging conditions, various structure directions contained in the pattern to be imaged are imaged with different contrast, so that, in the photoresist, different line widths occur from the various structure directions. This can be attributed, inter alia, to the fact that, at a reflective optical component, such as is normally used in catadioptric projection objectives for microlithography, light polarized perpendicular to the plane of incidence is reflected more strongly than light polarized parallel to the plane of incidence, that is to say the intensity ratio of s-polarized to p-polarized light at the exit from the projection objective differs from the intensity ratio at the entry. Since the deflection mirrors used in catadioptric systems are normally operated with large angles of incidence, at which the difference in the reflectance between s-polarized and p-polarized light is particularly high, considerable differences can occur in the intensity ratio of s-polarized to p-polarized light at the entry and exit from the projection objective.
Patent publication DE 198 51 749 A1 discloses a catadioptric projection objective in which effects dependent on polarization, which arise as a result of different reflection as a function of the direction of polarization and therefore produce differences in response and displacement of the position of the beam of rays producing the image on the wafer, are compensated for by matching dielectric reflex layers or by means of additional non-coplanar deflections. The disadvantage with the last-named solution is that it requires considerable expenditure on construction.
Patent application DE 102 40 598.0 from the applicant, which is not a prior publication, discloses an optical imaging system having a first and second reflection mirror, in which a ratio Rsp between the reflectance Rs of a deflection mirror for s-polarized light and the reflectance Rp of the deflection mirror for p-polarized light from a range of angles of incidence covering the associated tilt angle is greater than one in the case of one of the deflection mirrors and less than one in the case of the other deflection mirror. Such an apparatus can be used for the purpose of compensating for the polarization-changing action of one deflection mirror with the aid of the second deflection mirror.